The present invention relates to a crosslinkable silicone compound, stable under storage conditions.
The compound is used particularly but not exclusively in dentistry for preparing impression materials.
Conventional silicone rubbers are substantially constituted by silicone polymers and fillers.
They are mainly different according to the kind of polymer being used and to the cross-linking mechanism.
In materials for dental impressions based on addition-crosslinking polyorganosiloxanes, the crosslinking mechanism arises from the addition of silicon-hydrogen groups by means of double bonds in the presence of a catalyst based on platinum salts.
This reaction is known as hydrosilylation and occurs at room temperature when metal/olefin platinum complexes are used.
The setting reaction allows to obtain an elastomeric compound which has excellent characteristics.
Natural or synthetic mineral fillers, pigments, release agents, rheological modifiers et cetera are normally used in silicone rubbers.
For RTV (room temperature vulcanization) silicone rubbers used in dentistry for taking impressions, the product is usually marketed in two separate mixes, called "base" and "catalyst", which must be mixed uniformly before use.
After mixing, therefore, the compound is placed in contact with the surfaces to be duplicated until it sets completely.
Between the beginning of the mixing and complete crosslinking there is a first period, known as "working time", during which the two parts mixed together are plastic and can be worked, and there is a second period during which the mass loses its plasticity and setting begins.
The time during which the reaction occurs is known as "setting time"; at the end of this period, the product has become an irreversible elastomer.
To allow optimum clinical application, the total cycle up to setting must last a few minutes, usually between a minimum of one minute to a maximum of ten minutes from when mixing begins.
For users it is furthermore essential to have control over the reaction and to have a reaction in which performance always remains the same until the expiration date.
After setting, the product must exhibit some essential requirements for specific application in dentistry, such as dimensional stability, high detail reproduction ability, and as mentioned, constant working time and setting time.
The working time and the setting time can be affected by adding to the compound inhibitors which belong to the group of methyl or vinyl cyclotetrasiloxanes; apart from the possibility to control the reaction, it is very important that these parameters remain unchanged during the period of guarantee determined and specified by the manufacturer.
In order to ensure these conditions, the reactivity of the component which is most sensitive and therefore most unstable, i.e., the platinum catalyst, would have to remain constant; in practice, instead, it tends to lose part of its reactivity over time, degrading due to reasons which have not been properly clarified yet.
This becomes evident through an increase in the setting time, until the product becomes no longer suitable for use or no longer conforms to the specifications of the manufacturer.
This effect increases if the product is stored and especially if it is stored at high temperatures.